1. Field of the Invention
The present invention relates in general to the packaging of electronic assemblies. More particularly, it is directed to the manufacture of peripheral cards such as PCMCIA cards having overmolded housing and protection against both electro-static discharge (ESD) and electro-magnetic interference (EMI).
2. Description of the Related Art
The technological progress in personal computers has led to high demand for expansion boards to enhance their functions. Originally, these boards were unprotected printed circuit boards (PC boards) having edge connectors and carrying a variety of circuit components soldered thereto. These boards were installed inside the computer and therefore required the opening of the case of the computer. However, the advent of the laptop personal computers along with the rapid progress in miniaturization of integrated circuits have brought about the development of the portable peripheral card. A particular type of peripheral card known as PCMCIA card has emerged as the industry standard. Many of the current laptop and notebook computers contain built-in support for PCMCIA cards. PCMCIA cards are inserted in external slots of a laptop or notebook computer without the inconvenience of opening the computer.
Even though originally developed for memory expansion cards (the acronym "PCMCIA" stands for "Personal Computer Memory Card International Association"), the PCMCIA standard has since seen wide adoption in a variety of applications, such as FLASH memory cards, hard disks, FAX/modem, security cards, etc. In many of the applications, such as security cards where the card is carried with the card holder in much the same way as a credit card, susceptibility to ESD is of primary concern. Furthermore, as these peripheral cards are packed in with more complex circuitries, adequate shielding against EMI also becomes of great importance.
Electro-static charge may build-up on any surface area of a peripheral card. If this charge is not properly discharged, it can cause damage to both the electronic components inside the card and the computer to which the card is coupled. Therefore, an effective discharge path is needed from any surface area of the card.
To adequately shield the peripheral card against radiating or receiving EMI, the shield must have a low enough resistivity so that no electro-magnetic energy can penetrate. Therefore, the shield must have a combination of adequate thickness and conductivity. For example, a relatively thin continuous metal shield can provide adequate shielding at all frequencies. A polymer filled with conductive material, hereinafter referred to as filled polymer, on the other hand is likely to provide poor shielding at particular frequencies. Furthermore, the shield needs to provide a continuous conductive surface area to ensure uninterrupted surface current flow. Any discontinuities in the shield leads to what is called a slot antenna, through which EMI can penetrate.
A number of packaging techniques are available in constructing peripheral cards such as PCMCIA cards, some of which have been modified to provide limited protection against ESD and EMI.
One common packaging technique has been the use of sheet metal housing. FIG. 1 depicts a PCMCIA card constructed in accordance with this technique. A PC board 50 is electrically coupled to a female PCMCIA edge connector 51. The PC board 50 fits in a groove or shelf in a plastic rim 52 surrounding the PC board 50. Sheet metal jackets 53 are attached to the top and bottom of the assembly to form the complete PCMCIA card. Since the metal jackets 53 are usually fairly flexible, sheets of plastic insulators 55 are placed between the PC board 50 and the metal jackets 53. The plastic insulators 55 preclude the PC board 50 from coming into electrical contact with the metal jackets 53, thereby preventing electrical shorts.
ESD protection is provided by electrically coupling the metal jackets 53 to a ground trace on the PC board via conductive clips. The conductive clips are attached to ground pads on the PC board. These clips are raised from the PC board surface to make electrical contact with the metal jackets 53. Therefore, an electrostatic discharge path is provided from the metal jackets 53 to the ground circuit on the PC board via the conductive clips.
In protecting against EMI, the two metal jackets 53 provide adequate shielding in the surface area. However, where the two metal jackets 53 are butted together a slot antenna is formed through which EMI can penetrate. Several techniques have been used to ensure full electrical contact between the metal jackets 53 however, none have proven effective. Additional conductive clips are used to short the two halves of the case together to reduce the length of the slot antenna, thereby reducing the magnitude of the EMI problem.
The above packaging technique however, possesses a number of drawbacks. First, the manufacturing process must align and attach several parts: the PC board, the edge connector, the plastic rim, the conductive clips, the sheets of plastic insulators and the metal jackets. This multistage assembly process is time consuming and labor intensive and therefore quite costly. Second, the sheet metal jackets are generally fairly flexible implying a relatively weak external protective structure for the PC board. Third, the metal jackets do not fully protect against EMI since a perfect electrical contact may not be obtained between the two metal jackets, and the conductive clips will not fully eliminate EMI. Fourth, given the ease with which the metal jackets can be pried open, the ICs housed inside are easily accessible. This is of concern in applications such as security cards, where the ICs carry proprietary data.
Another method of packaging Peripheral cards is disclosed in the aforementioned patent document of Takiar et al., incorporated herein by reference. This method provides a solid one-piece injection molded package, housing the PC board. This method significantly simplifies the manufacturing process, thereby reducing the cost. Furthermore, the solid one-piece package provides a stronger external protective structure for the PC board as well as an added level of security for the ICs housed inside the card (i.e. the package can not easily be pried open).
Even though this method removes some of the drawbacks of the prior art, it however introduces others. This method requires the use of non-conductive molding material such as thermoset plastic, and such non-conductive housing renders the peripheral card and the ICs inside susceptible to both ESD and EMI.
From the above, there has been a need for a method of manufacturing peripheral cards such that the resulting card would have a solid housing replacing the several parts, while ESD and EMI protection are not compromised.